Antihistamines are a class of drugs commonly used to treat symptoms of allergies. These drugs help treat conditions caused by too much histamine, a chemical created by your body’s immune system. Antihistamines are most commonly used by people who have allergic reactions to pollen and other allergens. They are also used to treat a variety of other conditions such as stomach problems, colds, anxiety and more.

What are allergies?

Your body protects you from many threats. Your ribs protect your heart and lungs from injury. Your skin protects your body from outside elements like sun, wind and bacteria that can cause disease and infections. Your eyelashes protect your eyes from debris. And your body’s internal protection system – your immune system – battles substances that enter your body that are deemed “foreign.”

An allergy occurs when your immune system overreacts to the “foreign” substance. In the case of an allergy, substances that are usually harmless and don’t bother some people, such as dust or animal dander, do bother you! Your body views these substances as “foreign,” which then triggers an overreaction by your body’s defense system that includes the release of histamine. The substances that trigger the overreaction are called allergens. The symptoms that result are called an allergic reaction.

Allergies are one of the most common chronic conditions in the world. Some 40 million to 50 million people in the United States have them.

What is histamine?

Histamine is an important chemical that has a role in a number of different bodily processes. It stimulates gastric acid secretion, plays a role in inflammation, dilates blood vessels, affects muscle contractions in the intestines and lungs and affects your heart rate. It also helps transmit messages between nerve cells and helps fluids move through blood vessel walls. Histamine is also released if your body encounters a threat from an allergen. Histamine causes vessels to swell and dilate, leading to allergy symptoms.

What are some of the substances, or allergens, that cause allergies?

The top eight most common things that can cause an allergic reaction in some people include:

• Food.
• Dust.
• Pollen.
• Pet dander, saliva or urine.
• Mold.
• Insect bites and stings.
• Latex.
• Certain medications/drugs.

What allergic symptoms do histamines cause?

Too much histamine, caused by your body being oversensitive and overreacting to an allergen, causes a variety of symptoms. Symptoms include:

• Congestion, coughing.
• Wheezing, shortness of breath.
• Tiredness (fatigue).
• Itchy skin, hives and other skin rashes.
• Itchy, red, watering eyes.
• A running or blocked nose, or sneezing.
• Insomnia.
• Nausea and vomiting.

What are antihistamines?

An antihistamine is a prescription or over-the-counter medication that blocks some of what histamine does. “Anti” means against, so antihistamines are medicines that work against or block histamine.

How are antihistamines classified?

Antihistamines are divided into two major subtypes. The first subtype is called H-1 receptor antagonists or H-1 blockers. This subtype of antihistamines is used to treat allergy symptoms. The second subtype is called H-2 receptor antagonists or H-2 blockers. They are used to treat gastrointestinal conditions, including gastroesophageal reflux disease [GERD] (also called acid reflux), peptic ulcers, gastritis, motion sickness, nausea and vomiting. The naming structure (H-1 and H-2) tells doctors and scientists the cell type the location of the histamine receptor that the antihistamine medication blocks.

The H-1 blocker subtype is further broken down into two groups — first-generation antihistamines and second-generation antihistamines.

What’s the difference between first- and second-generation antihistamines?

Just like the name implies, the first generation antihistamine were the first type approved by the Food and Drug Administration (FDA). They began to be approved in the United States in the 1930s and are still prescribed today.

They work on histamine receptor in the brain and spinal cord along with other types of receptors. Most notable about this generation of antihistamines is that they cross the blood-brain barrier, which results in drowsiness.

Second-generation antihistamines were approved by the FDA and first came to market in the 1980s. The second-generation antihistamines do not cross the blood-brain barrier to the extent that first-generation do and therefore do not cause drowsiness at standard dosage levels. Second-generation antihistamines are considered to be safer than first generation antihistamines because they don’t cause drowsiness and interact with fewer drugs.

What are some examples of H-1 first- and second-generation antihistamines and H-2 blockers?

There are many prescription and over-the-counter H-1 antihistamines. If you have allergies, you’re likely taking a H-1 antihistamine. A few examples of first-generation over-the-counter and prescription H-1 blockers include:

• Brompheniramine (Children’s Dimetapp Cold®).
• Chlorpheniramine (Chlor-Trimeton®).
• Clemastine (Dayhist®).
• Cyproheptadine (Periactin®).
• Dexchlorpheniramine Dimenhydrinate (Dramamine®).
• Diphenhydramine (Benadryl®).
• Doxylamine (Vicks NyQuil®, Tylenol Cold and Couth Nighttime®).
• Hydroxyzine (Vistaril®).
• Phenindamine (Nolahist®).

A few examples of second-generation over-the-counter and prescription H-1 blockers include:

• Azelastine (Astelin®).
• Loratadine (Claritin®).
• Cetirizine (Zyrtec®).
• Desloratadine (Clarinex®).
• Fexofenadine (Allegra®).

If you’re taking an antihistamine to help with stomach issues, you’re likely taking a H-2 antihistamine. A few examples of H-2 antihistamines include:

• Cimetidine (Tagamet HB®).
• Famotidine (Pepcid®).
• Nizatidine (Axid®).
• Ranitidine (Zantac®).

Besides allergies, what other medical conditions do antihistamines treat?

H-1 antihistamines treat:

• Allergic rhinitis/hay fever.
• Allergic conjunctivitis.
• Hives and other skin rashes.
• Colds.
• Food allergies.
• Hypersensitivity to certain drugs.
• Insect bites and stings.

First-generation H-1 antihistamines also treat:

• Insomnia.
• Motion sickness.
• Anxiety.

H-2 antihistamines treat:

• Heartburn.
• Gastroeophageal reflux disease (GERD).
• Duodenal and gastric ulcers.
• Zollinger-Ellison syndrome.

Other conditions antihistamines treat include:

• Anorexia.
• Headaches.
• Anaphylaxis.
• Vertigo.
• Parkinson’s disease (to decrease stiffness and tremors).
• Some types of bone pain.

Your healthcare provider may prescribe antihistamines for even other conditions.

What are the side effects of antihistamines?

You and your healthcare provider should discuss specific antihistamines and decide together if the potential benefits of an antihistamine outweigh its potential side effects.

Some of the common side effects of first-generation antihistamines include:

• Drowsiness.
• Dry mouth, dry eyes.
• Blurred or double vision.
• Dizziness and headache.
• Low blood pressure.
• Mucous thickening in the airways.
• Rapid heart rate.
• Difficulty urinating and constipation.

Some of the common side effects of second-generation antihistamines include:

• Headache.
• Cough.
• Tiredness.
• Sore throat.
• Abdominal pain or discomfort
• Nausea or vomiting.

Common side effects of H-2 antihistamines include:

• Drowsiness.
• Joint or muscle pain.
• Headache.
• Confusion in the elderly.
• Dizziness.
• Breast swelling and tenderness.

In what dosage forms are antihistamines available?

Antihistamines come in several forms including:

• Liquids.
• Lotions.
• Syrups.
• Gels.
• Eye drops.
• Tablets.
• Nasal sprays.
• Creams.
• Capsules.
• Suppositories.

How do I know which antihistamine to take?

Because there are so many antihistamine products, both over-the-counter and prescription, and because they are used to treat so many different conditions, you may need help figuring out which medication to take. For minor ailments, you can probably take over-the-counter products. You can read the package labeling and match your symptoms to the labeled symptoms. Also, never hesitate to ask the pharmacist. They are highly schooled in the actions and effects and side effects of drugs. You may need to try different antihistamines (but no more than one at a time unless directed by your physician) to find the best medication to manage your symptoms.

If you need a prescription antihistamine, you and your healthcare provider will work together to figure out what medication will be best for you. Many drugs interact with antihistamines, so your healthcare provider will want to know what medical conditions you have and medications you are currently taking. They will also want to know if you are pregnant, plan to become pregnant or are breastfeeding. Some antihistamines are not recommended in pregnancy because they may cause birth defects in very high doses. Antihistamines can pass into breast milk, so you should consult with your healthcare provider before using antihistamines if you are breastfeeding.

Children and the elderly are more sensitive to the effects of antihistamines, so special consideration will be given to the use of these products in these patients. Never give over-the-counter cough and cold antihistamines to children under four years of age. These medications can cause life-threatening side effects.

Can antihistamines cause fever?

Fever is not one of the side effects of antihistamines.

Can antihistamines cause constipation?

Yes, some antihistamines, such as diphenhydramine, do cause constipation as a side effect.

Can antihistamines cause dizziness?

Yes. Dizziness is a common side effect of some antihistamines.

Can antihistamines cause depression?

One study of 92 people with chronic itchiness saw that patients who took the antihistamines cetirizine and hydroxyzine reported an increase in depression and anxiety. The effects of all antihistamines on mood disorders have yet to be studied.

Can antihistamines cause high blood pressure?

If you’re already taking medication for high blood pressure, combining that with an antihistamine can increase your heart rate and raise your blood pressure. Talk to your healthcare provider about your options.

Can antihistamines cause weight gain?

Antihistamines can cause you to gain weight, yes. One antihistamine, cyproheptadine, is used for that reason. Histamine is known to reduce your appetite, so antihistamines cancel that out.

What antihistamines can you take together?

Antihistamines should not be combined unless directed to do so by your healthcare provider under their guidance and supervision. Antihistamines should be used only as directed or you could experience serious side effects. Read labels very carefully.

What should I do if antihistamines don’t work?

Talk to your regular healthcare provider, your pharmacist or get an allergist to help you find ways to treat your allergies. Some allergies can be treated with decongestants or immunotherapy.

Can I take antihistamines if I’m pregnant or breastfeeding?

It’s safest to talk to your healthcare provider if you are pregnant, planning to become pregnant or are breastfeeding. Animal studies have shown that some antihistamines can cause birth defects. Small amounts of antihistamines pass on to your baby if you breastfeed. For these reasons your healthcare provider will want to talk with you and make careful choices (or different choices) if there is any concern for your or your child’s safety.

Are antihistamines safe for dogs?

Diphenhydramine is a common medication used to treat allergies, hives, food allergies, anxiety and other conditions in dogs. However, you should consult your veterinarian about the use of diphenhydramine in your pet. The dosage in dogs is based on their weight plus your veterinarian will want to examine your dog to be sure an antihistamine is the correct drug for the correct diagnosis. If an antihistamine is needed, your veterinarian will want to prescribe a brand that is specific to animals and at a dosage correct for your pet.

Do antihistamines cause dementia?

Long term use of some antihistamines may increase your risk of dementia. Diphenhydramine (Benadryl®) blocks the effects of a neurotransmitter called acetylcholine. This neurotransmitter is vital for memory and learning. Diphenhydramine increased the risk of dementia by 54% in one 3,000 patient study followed for seven years.

What questions should I ask my healthcare provider?

• What type of antihistamine would work best for me?
• How do I proper take the prescribed antihistamine?
• What side effects might occur with the recommended medication?
• What antihistamine won’t interfere with the current medications I am taking?
• When, or for what conditions, does taking an antihistamine that would make me drowsy make sense?
• Can I live my life normally while using this medication? Can I drive? Can I operate heavy machinery?
• Can I take antihistamines if I am pregnant, planning to become pregnancy or am breastfeeding?
• Can antihistamines be safely given to my child?
• What are the consequences if I don’t take an antihistamine to help with my allergies?

A note from Cleveland Clinic

Histamine is on your side. The chemical does its best to regulate help your heart and lungs and protect your body from foreign allergens, among other roles. But it can be oversensitive, and it can overreact, and that’s where antihistamines can help. If you’re have allergies, stomach symptoms or any of the other conditions and symptoms mentioned in this article, talk to your healthcare provider about your options. Your symptoms may be able to be treated.

Always check with your healthcare provider and your pharmacist if you have concerns about antihistamines, and always follow the directions on the labels!